The brightest A-level students could be deprived of an offer from top universities this year because their teachers are being given mixed messages over whether to predict the new A* grade, it emerged today.

Ministers have introduced the grade to help leading universities pick out the most academically able pupils from the growing number who achieve straight As.

But Oxford, Cambridge, Warwick and other leading universities have different views over whether teachers should predict the grade.

Speaking at the Headmasters' and Headmistresses' Conference, organised by an association of 250 private schools including Eton and Harrow, Oxford's director of admissions, Mike Nicholson, said that until teachers and admissions tutors understood exactly what standard was needed to achieve an A*, it should not be predicted.

Nicholson said Oxford tutors should ignore the grade if it was predicted and would not demand an A* until 2012.

"We are being very, very careful on this based on feedback from schools and colleges," he said.

Cambridge's director of admissions, Geoff Parks, told the headteachers he welcomed the new grade and said it would help universities to pick out the most talented students.

"We hope [the A*] will be a fairer system because those who get into Cambridge will have higher grades than those who don't," he said.

Michael Whitby, pro-vice-chancellor at Warwick University, said he would encourage teachers only to make A* predictions for students who hoped to read maths or science at university. It was easier to tell who would achieve the grade in these subjects than in arts and humanities, he said.

Teachers have until 15 October to make predictions for their students applying to courses at Oxford or Cambridge or for medicine or dentistry degrees elsewhere.

Cambridge and Imperial College London are asking for at least one A*, while Oxford and other universities will adhere to the government's guidance and not demand the new grade.

Tim Hands, the headteacher of Magdalen College, an independent boys' school, said teachers were confused by the different views of universities. He said he doubted admissions tutors would be able to ignore A* predictions.

"Can you really tell me that any tutor is going to look at a list of A* predictions and not think that that candidate is better than the one with four As? Universities should be transparent about what they are doing," he said.